As a printing method of forming a printing image on a printing medium on the basis of the signal of image data, there are an electrophotographic method, sublimation type and melting type heat transfer methods, and an ink jet method.
An electrophotographic method requires a process of forming an electrostatic latent image by electrostatic charge and exposure on a photosensitive drum, thus the system is complicated and an expensive apparatus is necessary.
A heat transfer method, although the apparatus is inexpensive, uses an ink ribbon hence running cost comes expensive, leaving waste materials behind.
On the other hand, in an ink jet method, as printing is performed directly on a printing medium by ejecting ink on only a necessary image part with an inexpensive apparatus, coloring materials can be effectively used and the running cost is inexpensive.
As a method of applying an ink jet technique to a printing system, e.g., a method of installing an ink jet printing apparatus on a rotary printing apparatus, and additionally printing variable numbers and marks on the same printing paper by an ink jet system is disclosed in JP-A-10-286939 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
However, it is more preferred that an ink jet printing technique is capable of printing high-degree image data like a photographic image. However, in an ink jet technique which ejects a water base ink or an organic solvent base ink containing conventional dyes or pigments as the coloring materials by making use of pressure, since droplets containing a large amount of solvent are ejected, a printed image blurs if expensive special printing paper is not used.
Therefore, when printing is performed on usual printing paper or non-absorptive plastic sheet, a high quality printed image cannot be obtained.
Further, as one of ink jet techniques, there is a method of forming an image which comprises a step of heat-melting an ink which is in a solid state at normal temperature and ejecting the liquefied ink. Blurring of printed images can be reduced with this ink, but the ejection of minute droplets is difficult since the ink viscosity at ejecting time is high, and each dot image obtained is large in area and thick, thus a highly precise image cannot be formed.